Cutting edge work on offshore integration

Time is tight and solutions many and varied to the problems posed by the installation of 80 Vestas 2 MW turbines in the North Sea off the exposed Danish west coast. Aside from the need for construction ingenuity, learning how to schedule 160 MW of wind output is also keeping system operator Eltra busy

With less than a year left before the first turbines go on-line at the 160 MW Horns Rev wind plant off the North Sea coast of Denmark inventive solutions to original problems have become the order of the day. From devising helicopter landing platforms, to construction of the first ever sea based high voltage sub-station, to first time connection of wind turbines directly into a 150 kV transmission line, to working out how to fix vessels to the seabed for tower and foundation installation, the problems seeking solutions are many and varied.

Three companies are deeply involved in development of the project, one of a series of five offshore plant ordered by the Danish government. Regional utility Elsam is to build and operate the plant together with turbine supplier Vestas, the independent transmission system operator, Eltra, is taking the power, and Tech-Wise, a 100% owned subsidiary of Elsam, is the project consultant. "Inventiveness has been necessary and because of the tight schedule we've hit on a number of tailor made solutions to specific problems. But both the experience gained and the actual technical solutions can in many cases be used again," says Jens Bonefeld of Tech-Wise, with an eye to future business opportunities, but also in recognition of the fact that specific solutions will often be more expensive than the off-the-shelf variety.

The Horns Rev site has an exceptional resource, with wind speeds averaging 9.7 m/s. Over a year, the plant is expected to notch up the equivalent of 3500 operating hours at full capacity -- 50% more than on land and an effective 40% capacity factor, compared with the normally accepted capacity factor for a wind plant of 26%. (Thermal plant operate at capacity factors 65-85%). On the negative side, gusts of up to 58 m/s have been registered at the site as recently as winter 1999 -- tough survival conditions for wind turbines -- and the plant will have to turn a profit while being paid the lowest tariff yet for renewables capacity in Denmark, says Flemming Thomsen, who heads Elsam's ten strong wind division. The tariff -- a minimum of DKK 0.453/kWh -- is to be made up of DKK 0.33/kWh for the first 44,000 full load hours (about ten year's of operation), plus the return on sales of green certificates, which have a politically set floor of DKK 0.10/kWh, plus compensation for unfairly imposed "imbalance penalties" of DKK 0.023/kWh.

Construction techniques

New thinking is needed on all fronts. Service and maintenance of the plant once it is running requires training of technicians, not only in turbine operation, but also in sea rescue and survival. The turbines can only be reached by boat in winds of less than 10 m/s, resulting in an average waiting time of three to three-and-a-half days before a malfunctioning machine can come on-line again. As a result, availability of the entire plant for power production is expected to be 85-95%, considerably less than the 98-99% now the norm on land. Once winds are blowing at over 8 m/s and whipping up waves, an alternative option is to use helicopters. Landing platforms are being considered for each of the 80 Vestas 2 MW turbines and for the electrical sub-station, or technicians will be lowered to the turbines from helicopters. Once winds are blowing at over 20 m/s, even the helicopters are grounded.

Construction of wind plant foundations and erection of turbines at sea is creating a whole new industry. Existing installation techniques and equipment developed by the offshore oil and gas industry are designed for far larger building work, while other existing solutions are not up to the job, says Bonefeld. Instead the project team are adapting known techniques and inventing new. A specially adapted pram will transport six foundations at a time to the site. It will be "parked" on four legs dropped to the seafloor at each turbine location -- a practice borrowed from the offshore platform business -- and the pram's hydraulic hammer will ram the foundations 25 metres into the seabed. The turbines will arrive in an adapted container ship complete with crane, which will be anchored using the same four-legged method.

Grid challenges

Construction of a high voltage substation at sea -- to receive power from a wind farm at 150 kV -- are both first-time occurrences for the electricity industry. Neither has been tried before. Indeed, according to engineer Jens Peter Kjærgaard from Eltra, the whole project means new challenges for the system operator. "It must be possible to regulate output from the turbines if there are problems with the grid or if there is too much electricity in the system, in other words if the grid on land is overloaded," he says. "In principle we will not interfere with production from the turbines, but we will set criteria for the maximum capacity that we can receive, because capacity regulation must be possible." Discussions on the criteria are now ongoing between all the parties.

As Denmark's big offshore wind farms come on-line, existing thermal plant will be retired and their job of maintaining grid frequency at 50 Hz could become impossible for Eltra. As a result, Eltra is looking to transfer some frequency control to the country's decentralised CHP plant and is currently working on technical solutions.

Electricity overspill -- power produced in excess of requirements -- is a problem growing day by day, according to Eltra. Wind tends to get much of the blame since it cannot be turned off, but the demands on decentralised CHP for heat actually cause most of the problem (table). Some of the overspill can be exported and some of it -- referred to as the "critical overspill" -- cannot be. Experts under the Danish energy ministry are now discussing solutions to the problem (story page 46). Eltra believes that demand management in the form of flexible time-dependent tariffs could solve much of the problem. Devising a tariff system which follows price movements on the electricity exchange "is a technique known in Norway and Sweden and only requires a political solution," says Eltra's Per Andersen.

Forecasting winds

Better weather (wind speed) forecasts could also help Eltra schedule wind output in its forward planning just as it schedules thermal plant to the daily swings in consumer demand for power. Current forecasting is for 24 hour periods, but is still not reliable enough, says Eltra. Together with Denmark's national laboratory at Risø, the Danish meteorological office and the technical university, the utility is working on a better forecasting model. If all goes according to plan the model should be able to forecast winds up to 48 hours ahead with an error margin of under 10% of installed capacity for two-thirds of the time.

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